netdev - Re: Regarding tx-nocache-copy in the Sheevaplug

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Message-ID: <1413413127.17186.5.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 15 Oct 2014 15:45:27 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Benjamin Poirier <benjamin.poirier@...il.com>
Cc:	Lluís Batlle i Rossell <viric@...ic.name>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Carles Pagès <page@...ata.homelinux.net>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Regarding tx-nocache-copy in the Sheevaplug

On Wed, 2014-10-15 at 14:57 -0700, Benjamin Poirier wrote:
> On 2014/10/13 12:52, Lluís Batlle i Rossell wrote:
> > Hello,
> > 
> > on the 7th of January 2014 ths patch was applied:
> > https://lkml.org/lkml/2014/1/7/307
> > 
> > [PATCH v2] net: Do not enable tx-nocache-copy by default
> >         
> > In the Sheevaplug (ARM Feroceon 88FR131 from Marvell) this made packets to be
> > sent corrupted. I think this machine has something special about the cache.
> > 
> > Enabling back this tx-nocache-copy (as it used to be before the patch) the
> > transfers work fine again. I think that most people, encountering this problem,
> > completely disable the tx offload instead of enabling back this setting.
> > 
> > Is this an ARM kernel problem regarding this platform?
> 
> This is odd, only x86 defines ARCH_HAS_NOCACHE_UACCESS. On arm,
> skb_do_copy_data_nocache() should end up using __copy_from_user()
> regardless of tx-nocache-copy.

 kmap_atomic()/kunmap_atomic() is missing, so we lack
__cpuc_flush_dcache_area() operations.





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